Find the "CSS Class" field and remove size-thumbnail, size-medium, or size-large

Click update and save the post

Alternately, you can control the image sizes using the Settings > Media page in your dashboard, but note those settings will affect all of the images on the blog using the same size names in the advanced settings tab.

but removing 'size-medium' does indeed fix the problem. Thanks you very much!

Awesome!

There is no "advanced settings" tab, mine is called "text"

The advanced settings tab only appears when you click an image and then click the edit button that appears on it. Here's an example: https://cloudup.com/cheNMDVY5UY

Now all the the programmers have to do is fix the *actual* bug that is causing this to happen.

It's not a bug, it's a change in the way the main media settings interact with blogs. There was one bug happening with the sizing for portrait images when we first updated, but that has already been fixed. There's another bug for captioned images, and that is being worked on and the code is still under review. You can follow updates for all the related issues at http://en.forums.wordpress.com/topic/images-resizing?replies=4 too.

When I resize an image, to 408px, and it appears always as 408px width and with script text that says "=480" in edit mode, but then the page appears live as a smaller image with the tags "=300", then THAT IS A BUG.

Thanks for telling me how to remove the "small-medium" text placed there by... THE BUG. A workaround is good enough. Fixing the actual problem would be even better.

@zenid10, I think it's a transition rather than a bug :) but I see your point. Also, a bug is something that is a mistake and this is more of an intentional change that we know will take a little bit of work to switch to but we feel that the benefits in the long run will work out in everyone's favor.

Sorry it's been causing trouble for you!

I have a tool now that will let me remove all size-thumbnail, size-medium, and size-large class name from the images—doing that will disconnect the sizes on the Settings > Media page from your images so you can't use the Settings > Media options to resize already-embedded images any more and so the settings won't affect past images you embedded in the past. Does that sound like something you'd like to do?